Oxidation Numbers and Ionic Formulas

• Many ionic compounds are binary meaning they only contain 2 elements

• ​What are some other things that are binary?

• the charge on an ion is known as its oxidation number

• it is equal to the number of electrons that the ion gained or lost

  Because atoms gain or lose electrons until they become stable, the oxidation number for many elements in the periodic table can be PREDICTED!​

Oxidation Numbers

Why is the cation attracted to the anion?

Criss Cross Rule

• Remember ionic bonds form when a cation is attracted to an anion.

• When the bond is formed, the compound is known as a formula unit, which can be represented through a chemical formula​

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What is always the charge of the ionic compound?

Formulas for ionic compounds

To write the formula for an ionic compound…

1. Find the oxidation number for each element

2. Reduce the oxidation numbers (if possible) to the lowest whole number ratio

3. Criss-cross the oxidation numbers

​Some text here about the topic of discussion.

sodium and chlorine

aluminum and sulfur

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nickel (II) and oxygen

Practice!

• ​Some ionic compounds contain more than two elements.

  These compounds contain a polyatomic ion, an ion with two or more atoms chemically combined.

  Polyatomic ions act as a unit

What is always the charge of the ionic compound?

Formulas for ionic compounds with Polyatomic Ions

To write the formula for an ionic compound…

1. Find the oxidation number for each element and polyatomic ion

2. Reduce the oxidation numbers (if possible) to the lowest whole number ratio

3. Criss-cross the oxidation numbers but never separate the polyatomic ion!!

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​Calcium and Phosphate